StackOverflow2013

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14793287
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2013-02-10T00:31:56.800
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Found the issue. Well, the tip of the iceberg. The code of the InternalOnNavigatedFrom method is: <pre><code>internal override void InternalOnNavigatedFrom(NavigationEventArgs e) { PhoneApplicationPage content = e.Content as PhoneApplicationPage; string str = ((content == null) || (content.Title == null)) ? string.Empty : content.Title; PerfUtil.BeginLogMarker(MarkerEvents.TH_ONNAVIGATEDFROM_PAGE, string.Format("{0},{1},{2}", (base.Title == null) ? "" : base.Title, e.NavigationMode, str)); this.OnNavigatedFrom(e); PerfUtil.EndLogMarker(MarkerEvents.TH_ONNAVIGATEDFROM_PAGE, string.Format("{0},{1},{2}", (base.Title == null) ? "" : base.Title, e.NavigationMode, str)); DeviceStatus.KeyboardDeployedChanged -= new EventHandler(this.OnKeyboardDeployedChanged); Task rootTask = ApplicationHost.Current.RootTask; rootTask.OnVisibleRegionChange = (ITask.VisibleRegionChanged) Delegate.Remove(rootTask.OnVisibleRegionChange, new ITask.VisibleRegionChanged(this.OnVisibleRegionChange)); Task task2 = ApplicationHost.Current.RootTask; task2.OnSipVisibilityChange = (ITask.SipVisibilityChange) Delegate.Remove(task2.OnSipVisibilityChange, new ITask.SipVisibilityChange(this.OnSipVisibilityChange)); this._lastSipHeight = 0.0; this._dictionary = null; } </code></pre> After a bit of debugging, I concluded that neither <code>e</code> or <code>Application.Current.RootTask</code> were null. After scratching my head, I looked at the code of the <code>KeyboardDeployedChanged</code> event handler: <pre><code>public static event EventHandler KeyboardDeployedChanged { [SecuritySafeCritical] add { if (KeyboardDeployedSubscription == null) { KeyboardDeployedSubscription = new SubscriptionHandler(DeviceTypes.KeyBoard); } KeyboardDeployedSubscription.Changed += value; } [SecuritySafeCritical] remove { KeyboardDeployedSubscription.Changed -= value; } } </code></pre> This code is poorly written. If the <code>remove</code> part of the handler is called before the <code>add</code>, <code>KeyboardDeployedSubscription</code> will be null and an exception will be raised. To test my theory, I subscribed to the event in <code>App</code>'s constructor: <pre><code> public App() { // Global handler for uncaught exceptions. UnhandledException += Application_UnhandledException; DeviceStatus.KeyboardDeployedChanged += (sender, e) => { }; </code></pre> And sure enough, the exception was gone. Now, to understand why your code is triggering this issue, I backtraced to which part of the framework is supposed to subscribe to the event. The only candidate is the <code>InternalOnNavigatedTo</code> method. Therefore, your issue is that <code>OnNavigatedFrom</code> is called even though <code>OnNavigatedTo</code> was never called.
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1. POOverriding WP8 Navigation - crash in PhoneApplicationPage
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1. PTAnswer
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1. USKevin Gosse
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2. VO
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